fix(cli): Handle the case when OS doesn't support symlink for adk run

Merge https://github.com/google/adk-python/pull/2582

Relate to #3306

## Description
Fixes issues #6 and #1785 where ADK commands crash on Windows due to symlink creation requiring admin privileges.

## Problem
On Windows, running ADK commands (like `adk run`) fails with `OSError: [WinError 1314] A required privilege is not held by the client: ...` because the logging system attempts to create a symlink for the latest log file. Windows requires administrator privileges for symlink creation by default (unless Developer Mode is enabled), causing crashes for non-admin users.

## Root Cause
The issue was in [`logs.py`](https://github.com/google/adk-python/blob/main/src/google/adk/cli/utils/logs.py#L72) where `os.symlink()` was called unconditionally without error handling, causing the entire CLI to crash when symlink creation failed.

## Solution
This PR implements graceful symlink handling. Changes made:
- Extracted symlink creation into separate function with error handling
- Added graceful fallback when symlink creation fails
- Replaced crashes with warnings to keep CLI functional
- Improved user messaging about log file locations

This solution follows a similar pattern to the one used in [ROS2](https://github.com/ros2) in its [logging module](https://github.com/ros2/launch/blob/rolling/launch/launch/logging/__init__.py#L93).

## Testing
### Before (broken)
```bash
> adk run my_agent
Log setup complete: C:\Users\username\AppData\Local\Temp\agents_log\agent.20250817_215119.log
Traceback (most recent call last):
  File "google\adk\cli\utils\logs.py", line 72, in log_to_tmp_folder
    os.symlink(log_filepath, latest_log_link)
OSError: [WinError 1314] A required privilege is not held by the client
```

### After (fixed)
```bash
> adk run my_agent
Log setup complete: C:\Users\username\AppData\Local\Temp\agents_log\agent.20250817_215319.log
UserWarning: Cannot create symlink for latest log file: [WinError 1314] A required privilege is not held by the client
To access latest log: tail -F C:\Users\username\AppData\Local\Temp\agents_log\agent.20250817_215319.log
Running agent my_agent, type exit to exit.
```

Co-authored-by: Wei Sun (Jack) <weisun@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2582 from lorenzofavaro:fix/windows-symlink-permissions 1e99a6287994f1bfd9ac11550d3dc06205998df5
PiperOrigin-RevId: 828690483

src/google/adk/cli/utils/logs.py

@@ -18,6 +18,9 @@ import logging
 import os
 import tempfile
 import time
+import warnings
+
+import click
 
 LOGGING_FORMAT = (
     '%(asctime)s - %(levelname)s - %(filename)s:%(lineno)d - %(message)s'
@@ -32,6 +35,38 @@ def setup_adk_logger(level=logging.INFO):
   adk_logger.setLevel(level)
 
 
+def _create_symlink(symlink_path: str, target_path: str) -> bool:
+  """Creates a symlink at symlink_path pointing to target_path.
+
+  Returns:
+    True if successful, False otherwise.
+  """
+  try:
+    if os.path.islink(symlink_path):
+      os.unlink(symlink_path)
+    elif os.path.exists(symlink_path):
+      warnings.warn(
+          'Cannot create symlink for latest log file: file exists at'
+          f' {symlink_path}'
+      )
+      return False
+    os.symlink(target_path, symlink_path)
+    return True
+  except OSError:
+    return False
+
+
+def _try_create_latest_log_symlink(
+    log_dir: str, log_file_prefix: str, log_filepath: str
+) -> None:
+  """Attempts to create a 'latest' symlink and prints access instructions."""
+  latest_log_link = os.path.join(log_dir, f'{log_file_prefix}.latest.log')
+  if _create_symlink(latest_log_link, log_filepath):
+    click.echo(f'To access latest log: tail -F {latest_log_link}')
+  else:
+    click.echo(f'To access latest log: tail -F {log_filepath}')
+
+
 def log_to_tmp_folder(
     level=logging.INFO,
     *,
@@ -64,12 +99,7 @@ def log_to_tmp_folder(
   root_logger.handlers = []  # Clear handles to disable logging to stderr
   root_logger.addHandler(file_handler)
 
-  print(f'Log setup complete: {log_filepath}')
+  click.echo(f'Log setup complete: {log_filepath}')
+  _try_create_latest_log_symlink(log_dir, log_file_prefix, log_filepath)
 
-  latest_log_link = os.path.join(log_dir, f'{log_file_prefix}.latest.log')
-  if os.path.islink(latest_log_link):
-    os.unlink(latest_log_link)
-  os.symlink(log_filepath, latest_log_link)
-
-  print(f'To access latest log: tail -F {latest_log_link}')
   return log_filepath